Mindboggling Science: The Leidenfrost Edition
Water flows uphill? Liquid nitrogen skitters across a table? A hand immersed in hot molten lead emerges unscathed? Are these cinematic tricks? Are the laws of physics being broken? Alas, science saves the day with a very simple explanation of these phenomena: the Leidenfrost effect.
One of the most common examples of the Leidenfrost effect in action involves a pancake griddle. Water droplets will hover over a very hot pancake griddle and take a long time to evaporate. The griddle is much hotter than the boiling point which causes the water near the griddle surface to rapidly turn to steam so that the drop hovers on its own vapor cushion. This vapor cushion insulates the water droplet from the hot griddle which slows overall droplet evaporation. Oddly, if the temperature were reduced below the Leidenfrost point, the droplet could touch the surface directly and rapidly boil away. This phenomenon is counterintuitive of course: total evaporation is slower on the hotter surface. It also causes difficulties as engineered cooling systems are expected to transfer more heat, rather than less, as surface temperatures rise.
Various aspects of the Leidenfrost effect have been studied since it was first reported by Johann Leidenfrost in 1756. Measurements typically have been performed with zero or small incident velocity. However, in many real-world situations droplets crash into heated surfaces which influences the Leidenfrost temperature and local heat transfer. Models need to be developed to help us understand and control processes such as rewetting of fuel rods in a nuclear power plant during reflooding after the postulated loss-of-coolant-accident where droplets in the dispersed flow regime impact hot cladding.
High-speed x-ray full field phase-contrast imaging, a relatively new technique, makes it possible to track dynamic changes in gas-liquid interfaces in real time. The partially coherent x-rays produced by a synchrotron such as the APS can reveal the interface between phases even when the interface is inside a thick liquid medium. This makes it possible to clearly visualize internal vapor generation within a falling droplet at unprecedented speeds and spatial resolutions. These visualizations will be presented and scrutinized during this talk.